Dont know about extending - havent tried yet - but I wanted to shrink C: down from 149Gb (only want the Opsys in that ptn) and DiskManagement & DISKPART wouldnt play despite there being 120+Gb freespace - GParted did the trick , took some time but all seems OK since... YMMV
The shrink partition utility can only decrease the partition size down to the first unmovable/locked system file. To get round this, temporarily turn off System Restore, do the shrink and turn System Restore back on. Running a defrag may also move some of the ‘unmovable’ files.
Alternatively, you can shrink partitions by running the diskpart command from the Vista DVD, which avoids the occurrence of locked files.